Alloys and electrical transducers



Aug- 24, 1965 A. KRINSKY 3,202,951

ALLOYS AND ELECTRICAL TRANSDUCERS Filed Aug. 16, 1961 IN V EN TOR.' lA/berf Kr/'ns/ry To Vac Pump By m vv-cmu A Homey VWhiclris notl limitedto drawn 3,202 951 ALLGYS AND ELECTRECAL TRANSDUCERS Albert Krinslry,Bethesda,Md.,gassignor to the United States of America as represented bythe United States Atomic Energy Commission Filed Aug. 1a, 1961, Ser. No.131,944- 8 Claims. (Cl. S33-2) The present invention relates to newalloys, to electrical 4resistance elements utilizingsuch alloys whichare particularly-adapted for use at elevated temperatures, and to newand improved electrical transducrs, including strain gauges andresistors, employing such elements.

. A desirable characteristic of Valloys to be used in high temperatureapplications is the ability to resist oxidation for prolonged periods.lThis property is of particular importance in alloys which are used inthe resistance element of electrical transducers, such'as strain gauges.

Electrical strain gauges'have come to be Widely used for ascertainingstresses and strains to4 which articles are subjected. In the past, theywere usually constructed with the resistance element in the form4 of atine wire disposed in back and forth manner in spaced apart parallellines, the parallel lines beingfbonded throughout their lengths to aninsulatingbase by some suitable adhesive. More recently, strain gaugeshave been constructed with the resistance element formed bybonding athinmetal foil to an' insulating backing, the resulting gauge being attached.to thearticle to be tested by a suitable adhesive.

The nature of thel backings and adhesives have limited previousstrain'gaugesto usage at relatively low temper-atures.V higher:temperatures Vby Idiscarding the referred to backings and attaching thewire or foil directly to the article to lbe tested by insulatingadhesives. presents disadvantages in that the line wires and foils arediiiic'ult to handle, particularly when` used vwithout an insulatingbackingA or a supporting base in the higher temperature applications. Afurther disadvantage or drawback is the necessity of using anon-conducting adhesive -whichwill not chemically attack orobjectionably aitect the tine wires `or foils. A further objection t-oprevious strain gauges is that they. have been limited to 'such metalsand'alloys as could be drawn or rolled to small diameters orthicknesses.

TheA present inventionis directed Vto an alloy having greatly" improvedhighY temperature oxidation resistance which may be yused `in electricalresistance elements exposed 'to oxidizing lconditions at'elevatedtemperatures and is particularlydirected `to providing a resistance ele-=ment to overcome the above andother diic'ulties and disadvantages foundin present` strain gauges, and a method of making strain gaugesutilizing the resistance element y n y or rolled wires or foilsrandwhich-may be readilyand satisfactorily employed in high temperatureapplications.

For the purpose of clarification of the present invention,

'thete-rm resistance element is used to describe a resistance lilinand`itsassociated backing,V togetherlwithfthe appropriate electrical leadsattached thereto. Thus, the resistance film-described `herein,.to beused in -a resistance element, requires some additional supportingmember in addition to the electrical leads.

An object to the'present invention is to provide noveliron-aluminum-titanium oxidation resistant alloys, suitable 6 for use inelectrical resistance elements.

Another object o f ,the invention is to provide new andimprovedelectrical. resistance elements. i n Another'objectoftheinv'ention is to provide electrical transducers which areparticularly, adaptable to use in high temperature applications;V v

It has been attempted touse such' gauges at` This latter procedureUnited States Patent() 3,202,951 Patented Aug. `24:, 1965 Another objectof Ithe Ainvention is to provide a strain gauge which is particularlyadaptable to use'in high temperature applications.

A further object of the invention is to provide a new and improvedmethodof manufacturing said strain gauge.

` A further object of the invention is to provide astrain gauge whichdoes not require employment of any wires 'in its strain measurementportion, either in the gauge 'ment of the invention in practice.

In laccordance with the present invention, I have found a novel alloy ofthe composition range of from 25-30 atomic percent iron, 3`5.501 atomicpercent aluminum, and 20-40 atomic' percent titanium. Alloys in thiscomposition range resist oxidation at'high temperatures, are notattacked by such corrosive.v agents as concentrated hydrochloric acid orwater,` and do not'tarnish'. These properties make the alloyparticularly desirable as a protective coating in corrosiveapplications. Alloys in the above 'composition range are also useful linthe form of a thin film inthe resistance element ofelectricaltransducers, such ias the strain gauge hereinafter described.-

Apreferred embodiment of a strain gauge employing the all-oy of thepresentinventi'on in theorm of a thin film in theresistahce elementalsodescribedas astrain sensitive film-has been chosen for purposes ofillustration and description. The preferred embodiment is not intendedto ybe exhaustive nor to limit theinvention to the precise formdisclosed. lIt is chosen and described in yor-der to best explain theprinciples of .the invention and ltheir application inV practical use tothereby enable Iothers skilled in the art to best utilize the inventionin various embodiments andmodiications as are best adapted to theparticular use contemplated. Y

In the .accompanying drawings, which are-more or less diagrammatic: k

fFlG. 1 is .a planV viewof `a preferredembodiment of fthe invention. p Y

PIG; 2 is an enlarged cross sectional view taken along line 2 2 ofFIG. 1. n

FIG. 3 is-a. schematic sectional view'illustrating the manufacture ofthe present strain gauge.

Referring moreI particularly to-FIGS. 1 and A2, there is shown asubstrate 1` carrying a strain sensitive iilm 2 electrically connecte-dwith anchored wire leads 7 by intermediate electrically conducting lil-mcontact-s 8. The substrate comprises a thin metal toil 4 Vhaving at eachdat face thereof a ceramic layer 5. The intermediate foil 4-may be of 'ahighftemperature corrosion resistant metal about .001 inch in thickness.

Suitable corrosion resistant metals include alloys of ironnickel, orchromium base types; for example, the nickel-base alloy Inconel havingthe following composition has given good results.

The ceramic layer may be of any suitable high temperature ceramic; thegenerally known National Bureau of Standards composition, NBS CeramicCoating A-4l8, has given good results. (Composition Vdescribed inCuthill, Richmond, and Tighe, Effect of a Ceramic Coating on the CreepBehavior of Some High Temperature Alloys, American Ceramic SocietyBulletin, 3S (1959), pp. 4-12, Tables 2 and 3).V ,The thickness of eachof the ceramic 4layers 5 carried by the foil layer may be about .001inch,

.making a total thickness of the substrateof about .003

inch. The resulting substrate 1 isl thin7 llexible, nonconducting andhas smooth non-porous surfaces; the substrate may readily be bent-around a very small radius or cut with shears without danger ofcracking or chipping lthe ceramic layers 5. The method of making thesub-y strate will be fully described hereinafter.

The leads 7 are embedded in the ceramic layer 5 at one side of the foillayer 4 and are thus iirmly anchored in position. Various metals may beused for the lead wires, e.g., platinum, platinum-10% rhodium alloy, ora nickel-chromium alloy such as 80% nickel-20% chromium. While wire ofany suitable diameter may be employed, a wire diameter Iof about .005inch has given good results.y n

The electrically conducting lilm contacts 8 may comprise gold orplatinum areaswhich overlap and electrically connect with theembedded'lead wires 7. A gold lm is preferred due to the much lowerresistivity of the deposited ilm.

The strain sensitive lilm 2 carried by the ceramic layer `5 extendsbetween and overlaps the electrically conducting film contacts 8 and maybe formed by evaporative deposition of an alloy of iron, aluminum andtitaniumon the substrate 1, aswill be hereinafter more fully described.While thethin strain sensitive lm 2 is shown of gen- `erally rectangularoutline or shape, this is purely for purposes of illustration asit maybe of any other shape, e.g., spiral, arcuate, etc., appropriate to itsemployment. The resistance of the-film, 2 may be controlled by varyingeitherk the thickness of the lm or its area, or both. Elec- -tricalstrain sensitivity over wide limits may be obtained Y or controlled ,bysuitably varying the' thicknessl of the film. The temperaturecoeiiicient of resistance may be either negative, or positive, or zero,depending on the pa'rticular lm thickness selected.

The strain sensitive iilm 2 is carried by a non-conducting ceramic layer5, and hence when the device is secured to' an article oitest structureit is velectrically insulated therefrom so that the adhesive or cementused to secure the` device in position need notbe of the electricallyin- 1 sulating type.V When the device is to be used under conditions oftemperature` below 200 C. epoxy type adhesivesmay be usedV and lforhigher temperature'uses I ceramic type adhesives may be employed.

The substr-ate'l may be'manufactured by suitably cleaning and degreasinga lengthy of lnconel foil and then dipping it into a slurry of the NBSA-4l8 ceramic, al-

lowing the deposited slurry to air dry and then tiring the coated foilinV arkilnY that is maintained at a temperature of about l900 F., forabout 10 minutes.v The resulting composite substrate 1 is thin,flexible, yand non-conducting with a smooth nonporous surface. Thethickness of the ceramic coating 5 on the intermediate foil 4-may becontrolled b'y adjusting they consistency of the slurry. As previouslyindicated, a total thickness of the substrate 1 of about .003 inchhasbeen found to be satisfactory for use under many conditions land withmany test articles..V Whiley the ceramic couldbe applied by painting orotherwise, the described dippingstep has been found to giveA an optimumcoating. v n

After preparation of the substrate 1 as above, the leads 7 maybeattached by embedding them in the ceramic coating. This may be achievedby placing an end portion of a lead wirev7 against the ceramic surfaceand heating the adjacent contact area with an oxygen torch until the nminutes.

ceramic melts and engulfs the end portion of the wire. When properlyexecuted, the lead Wires areA iirmly embedded in, or-bonded to, thesubstrate but are electrically insulated from the intermediate foillayer 4 of the substrate as they are not so deeply embedded as tocontact the layer. This described procedure is preferred to endeavoringto attach the leads prior to drying and firing ofthe ceramic slurry.

The next step in manufacture of the substrate 1 is to apply the filmcontacts 8. This may be done by painting the area with a suitableresinous suspension of a heat decomposable compound of an electricallyconducting, oxidation resistant metal such as gold or platinum. Thereare available such suspensions of compounds as Liquid Bright Gold forCeramics or Liquid Bright Platinum No. 05 madey by Hanovia Chemical andManufacturing Co., Newark, New Jersey,v or Liquid Bright Gold #4942 orLiquid Bright Platinum #6558 made by E. l. du Pont de Nemours & Co.Inc., Electrochemicals Dept., Wilmington, Delaware. These resins, inaddition to binders, contain a solution of a compound of the metal; theyare applied so as to overlap and make electrical contact with theembedded lead wires 7. The resin is then air dried and subsequentlyheated or fired at about 1300 F. so as to decompose the resin anddepositan adherent layer or lm of the metal (preferably gold orplatinum) from the resin which carried it. The other component of thecompound and the biners are volatilized by the heat.v

The thus prepared substrate is next cleaned as a preliminary toevaporative deposition of the strain sensitive film 2, under evacuationconditions. While any of various cleaning methods may be used, it hasbeen-found satisfactory to wash the substrate with a heated detergentand water solution, thereafter rinse in water, and finally immerse it inhot iso-propyl alcohol vapor for about 10 When removed from the vapor,the substrate is clean and dry. n

VThesubstrate is now ready to receive the strain sensitive lilm 2. Oneor more of the substrates'are supported (FIG. 3)r on a suitable mask 13so as Vto overlie openings 14 therethrough. The sizes and shapes of theopenings 14 are such as to form a film 2 of desired size andconguration. As shown, the mask and the substrates which it supports areheld above a heat source or evaporation boat 15 by a support rod 16which'is in turn secured to a base plate'or tablev17. n

The evaporation boat is lheld in position in any suitable manner and iselectrically connected with terminals 19 that lead to a high currenttransformer or the like (not shown), whereby the evaporation boat may bemaintained in heated condition (preferably 'about 1500. C.)

vby passing electric current through it, so that materials in a hopper20 which has a bottom or outlet orifice of such size that the powderdoes not normally flow through it but does flow therethrough when thehopper and contents are vibrated or agitated. When agitated, the powdermixture is released or falls through the `hop'perroriice into a deliverytubeYZl, from whence it is discharged onto the Yheat source orevaporation boat -15. The

hopper may be carried adjacent one end of an arm 22, which is mountedadjacent itsother end on a support column 23. Any suitable agitatingmeans, for example, an electrical buzzer 25, may be carried by orsecured to the arm 22rfor vibrating itand agitating the hopper to effectflow of powdertherefrom into the delivery tube 21.

A bell jar or cover 26 restsupon the base plate 17 and' encloses thedescribed combination, so that the interior of the bell jar may beevacuated via a conduit 26 which leads to a vacuum pump (not shown).-

` In manufacturing operations the chamber within the 5 bell jar 26is'preferably evacuated to a pressure of about *4 to 10-5 millimeters ofmercury or less, and the heat source or evaporation boat is brought to atemperature of about 1500" Cyby passing electric current therethrough.lThe hopper is then vibrated or agitated to release' thepowder mixturewhichcomprises the powders of iro'n, aluminum, 'and titanium in thevdesired proportions. The powder mixture, falling onto the evaporationboat 15 from delivery tube'21, evaporates almost instantly uponcontacting the `boat and, the vapors of the metals mixing in transit,condenses on the exposed area Hof the substrate'l as an alloyfilm,overlying and electrically contacting the electrically conductingfilm contacts 8.

The lalloy-film thus obtained has been found highly satisfactory-in hightemperature Aapplications when produced from powder mixtures with thecomposition range of 25930 atomic --percent iron, l35-50atomic percentaluminum, 4and20v.to-40-atomic percent titanium. The particular alloyywithin this-'range which has the optimum ability to resistoxidation athigh temperatures contains 27.3 atomic percent iron, 45.5 atomic percentaluminum, and 27.2 atomicpercent titanium. Alloys of these componentshaving the composition varied within the specified composition rangealso`have suitable characteristics for the present purpose, It has beenfound, however, that iron-aluminum-titanium alloys having one or more ofthe alloy constituents outside of the specified composition range areunsuitable for the present purpose since the oxidation resistance fallsoff rapidly. For example, an alloy containing 10 atomic percent iron,57.6 atomic percent aluminum, and 32.4 atomic percent titanium oxidizedalmost completely within a l5-minute period when heated in air at 400C., this oxidation evidenced by marked changes in appearance and a greatdecrease in electrical conductivity. The same result was experiencedwith films of the elemental metals iron, aluminum, or titanium. Incontrast, alloy lms within the specified composition range have beenfound to resist oxidation under the same conditions even after severalhundred hours exposure; little change occurs in either appearance orelectrical conductivity of these alloy films. The following table givesthe composition of some typical alloys within the specified range whichwill perform satisfactorily under the above conditions.

Alloy No. Composition (atomic percent) l a 27.5 Fe-49 Al-23.6 Ti 2 25Fe-SO Al--25 Ti 3 26 Fe-36 AL-38 Ti 4 25.4 13e-35.6 Al39 Ti 5 28.8 Fe-40Al-31.2 Ti 6 29 Fe-40 Al-3l Ti Thus, an important characteristic of thenew nonaluminum-titanium alloys of this invention is their ability towithstand oxidation under high temperature conditions for a prolongedperiod, far exceeding that of other known alloys of these meals or ofthe elemental metals iron, aluminum, or titanium.

While the alloy film remains oxidation resistant up to temperatures ashigh as 400 C., above that temperature there is a tendency for the filmto oxidize. However, even this upper temperature may be extended severalhundred degrees by coating the film with a protective coating of siliconmonoxide. The silicon monoxide may be evaporatively depositiedsubsequent to the previously described evaporative deposition of thealloy film without interrupting the existing vacuum condition in thechamber of the bell jar. Other characteristics of the alloy film add toits desirability for use in a resistance element. It is hard and hasexcellent adhesion to the ceramic layer 5. It has also been formed onglass, enamels, metals, etc., and in so far as can be ascertained, maybe removed only by abrasive grinding or chemical attack. This hardnessand adhesion, which has been cleanliness, contrasts markedly with therather soft and weakly adhering films obtained from each of theelemental metals iron, aluminum, or titanium, even on carefully cleanedsubstrates.

An important advantage of the described method for producing the straingauge is that it facilitates use with such metals or alloys as can notbe readily drawn or rolled to small diameters or thicknesses. Nopreliminary wire or foil is necessary as the various powders desired maybe placed in the hopper 20 and agitated and delivered to the heat source15 for immediatey evaporation and deposition on a desired substrate.

It will then be seen that the present invention provides a novel methodof makingfa strain gauge; the method is relatively simple in operationand may be performed with inexpensive and readily available equipment.The

lresulting strain gauge may be used at high temperatures, up toabout'400 C., in air without any protective coating on the strainsensitive alloy film and even higher with the protective siliconmonoxide coating applied to the film. The gauge may be bent around.articles of small radius and the substrate portion of the gauge may becut as desired, without cracking or chipping, in order to fit the gaugeinto particular applications. The wire leads are embedded in' thev gaugeso as to be integral with the elastic and non-conducting substrate.

The aforementioned oxidation and abrasion resistance of the alloy filmmakes it also usefiul as a high temperature resistor and potentiometersurface when it is deposited on a rigid, non-conducting backing. rPhenonconducting backing may be of ceramic, glass, or other hightemperature material. -For example, glass and ceramic plates such asCorning Fotoceram (commercially available from Corning GlassCorporation, Corning, New York) have been 'successfully used as backingmembers. Electrical leads and film contacts are then applied to thesebackings in the same 4manner as previously described for l[the stra-inga-uge. In certain resistor applications, the alloy film may beovercoated with the protective film of silicon monoxide to increase itshigh temperature oxidation resistance so that the maximum temperature atwhich the alloy may remain ruseful is several hundred degrees higherthan the maximum temperature attainable without the protectiveovercoating, i.e., 400 C. As previously mentioned, the shape of the filmmay be varied appropriate to its employment.

As various changes may be made in the form, construction and arrangementof the parts herein without departing from the spi-rit and scope of theinvention and without sacrificing any of its advantages, it is to beunderstood that all matter herein is t-o .be interpreted as illustrativeand not in a limiting sense.

IIclaixn:

=1. An alloy for use as a heat generating electric current conductingelement and as a protective coating subjected to heat consistingessentially of 25 to 30 atomic percent iron, 35 to 50 atomic percentaluminum, and 20 to 40 atomic percent titanium, characterized by resist-`ance to oxidation on exposure to air and subjection to concurrenttemperatures up to about 400 C.

2. An alloy for use as a heat generating electric current conductingelement and as a protective coating subjected to heat consistingessentially of 27.3 atomic percent iron, 45.5 atomic percent aluminum,and 27.2 atomic percent titanium characterized by resistance tooxidation and concomitant minimization of electrical conductivity changeon exposure to air and subjection to concurrent temperatures Iup toabout 400 C.

3. A strain gauge comprising the combination of a strain sensitive film,an electrical insulating substrate supporting said strain sensitive filmwith said substrate compri-sing a thin metal foil carrying andsubstantially surrounded by a ceramic coating, and leads embedded in theceramic coating at spaced locations electrically connected to -r saidstrain sensitive film.

4. The straingauge as claimed in claim 3, wherein said strain sensitivelm is a metal alloy consist-ing f essentially of '25, to 30 atomicpercent iron, 35v to 50 atomic percent aluminum, Iand 20 to 40 atomicpercent titanium,tcharacterized by resistance to oxidation on 5. Thestrain gauge as -claimed in claim 3, wherein said strain sensitive lm isa metal alloy consisting essentially of 27.3 atomic percent iron, 45.5atomic percent aluminum, and-27.2 atomic percent titanium, characterizedby f resistance to oxidation `and concomitant minimization of electricalconductivity change on exposure to Iair and subjection to concurrenttemperature up to about 400"` C.

6. The strain gauge as claimed in claim 3, wherein j each of the V leadsis electrically connected with the strain sensitive lm ,by` anintermediate electrically 1 conducting'` `film contact carriedA bythecoiating Van overlying .any embedded. lead portion.

7. The stnain gauge claimed in claim 3 wherein the `layer issubstantially equal to the thickness Y metal toil. v

8. The strain gauge claimed in claim 7 wherein the ".cqmbined .thicknessof the .thin man foil and the ce- Y Tamiclayers is labout .003inchenabling' theA resulting exposure to ,air and subjection toconcurrent temperal tures up to about 400" C.,

References Cited by the Examiner UNITED STATES PATENTS '2,464,836 3/49Thomas et al. 75-l24 X 2,548,592 4/51 De Michele.

2,739,212 3/'56l Woolley etal. t z2,926,325 l2/60 Moore et al. 338-3082,927,048 3/60 Pritikin i 3-3-8-308 {2,934,7361 4/60' Davis 338-3082,939,807 6/60 Needham 338-308 3,008,109 lil/6l Starr 338-2 3,056,937=1\0/62 Pritikin 338--308 3,067,310 12/62 Walz et al. 3384308 X M. KO.LYONS, Examiner.

of the thank

1. AN ALLOY FOR USE AS A HEAT GENERATING ELECTRIC CURRENT CONDUCTING ELEMENT AND AS A PROTECTIVE COATING SUBJECTED TO HEAT CONSISTING ESSENTIALLY OF 25 TO 30 ATOMIC PERCENT IRON, 55 TO 50 ATOMIC PERCENT ALUMINUM, AND 20 TO 40 ATOMIC PERCENT TITANIUM, CHARACTERIZED BY RESISTANCE TO OXIDATION ON EXPOSURE TO AIR AND SUBJECTION TO CONCURRENT TEMPERATURES UP TO ABOUT 400*C. 